1. Field of the Invention
The invention relates to the field of switch mode power supplies, and amongst other things to power supplies that provide multiple output power levels.
2. Background
With the ever-growing demand upon power systems by the use of electronic devices that consume power non-linearly, the amount of apparent power that needs to be delivered is greatly increasing. One of the major problems associated with non-linear power supplies is the harmonic distortion of the current in the power supply grid. That is, non-linear power supplies convert the AC mains signal to a DC signal which results in a non-sinusoidal current being injected into the power grid. The non-sinusoidal character of the current is measured both in terms of power factor of the power supply and the harmonic content of the current waveform.
The production of a non-sinusoidal current can be understood with reference to FIG. 1. In FIG. 1, the AC Mains voltage 5 is input through an Electromagnetic Interference (EMI) filter 10 and a bridge rectifier 15. Bridge rectifier 15 feeds a rectified voltage into capacitor 25 which is used to maintain a substantially DC voltage 17. DC voltage 17 is maintained constant except for a ripple component 27 (FIG. 2A) since there is a certain discharge from capacitor 25 due to the time constant (T) of the capacitor 25, is not sufficiently more than the inverse of the frequency of the AC mains voltage 5 to prevent some voltage discharge. As can be seen in FIG. 2A, the ripple component 35 will at certain times have a magnitude greater than the instantaneous magnitude of AC mains voltage 5. This will cause periods when no current flows through bridge rectifier 15 and the current 22 (FIG. 2B) at the input of bridge rectifier 15 will be discontinuous causing harmonic distortion and a poor power factor. A clamping circuit comprising a zener diode 30 and a diode 35 is used to protect MOSFET 100 from over voltages. Power supply regulation is performed using a flyback topology well known in the art.
The discontinuous current waveforms injected into the power grid can cause the neutral wiring to overload and burn. Further, the harmonic currents cause an under utilization of electrical distribution and generation equipment, thereby increasing the cost of power generation to the utility. Further problems associated with harmonic currents include errors in utility metering equipment, malfunctioning of utility relays, and interference with communication and control signals in nearby lines.
The problems associated with harmonic currents in utility systems have led to the formation of various national and international organizations that are directed toward creating standards to limit harmonic currents for various classes of non-linear power devices. One such organization is the International Electrotechnical Commission, which issued the IEC 1000-3-2 standard that calls for various maximum levels for harmonic currents. These harmonic currents are currents at frequencies that are whole number multiples of the AC mains line frequency.
The harmonic currents created by the circuit described in FIG. 1 is usually far in excess of the requirements of the IEC 1000-3-2 standards. Thereby, power supplies are being designed to improve the harmonic currents. The improvements generally include using added circuitry to decrease the current waveform distortion and to reduce the periods when there is no current flowing through bridge rectifier 15.
Referring to FIG. 3, a modified power supply circuit incorporating a separate power factor correction stage 120 along with a flyback stage 20 is depicted. The power factor correction stage 120 includes a switch mode power supply regulation chip 195, which can be a TOPSWITCH.RTM. device manufactured by Power Integrations, Inc. Chip 195 has three terminals, which includes a control terminal 196. The input into control terminal 196 is a current 200 which is a combination of a current 191 which is proportional to the instantaneous rectified voltage and a feedback current 192. Feedback current 192 will be zero until the voltage of capacitor 185 reaches a voltage greater than the reverse break down voltage of zener diodes 150 and 155. In a typical power supply this voltage is typically 400 volts. As the current 200 increases, the duty cycle of the MOSFET in chip 195 decreases.
The MOSFET current 198 is comprised of series of triangular pulses each having a duty cycle. The frequency of triangular pulses, which generally ranges between 25 and 200 kHz, is very high compared to the frequency of a half wave rectified AC Mains voltage 200 which is at 100 Hz (2.times.50 Hz) or 120 Hz (2.times.60 Hz). The average MOSFET current 198 that is the average of the triangular pulses is both continuous and a distorted sinusoid when viewed over a single period of the 100 Hz rectified AC mains signal. The diode current 199 is also a series of triangular pulses, the pulses being complimentary to the triangular pulses of the MOSFET current 198. Therefore, average diode current 199 is also both continuous and a distorted sinusoid when viewed over a single period of the 100 Hz half wave AC mains signal.
The average input current waveform 197 at the input of bridge rectifier 15 is kept substantially sinusoidal by allowing current to flow through the MOSFET that has a shape which forms a complete sinusoidal current waveform when added to the average diode current 199 flowing through the boost diode 155 as can be seen in FIG. 4.
Capacitor 25 filters both the high frequency currents due to device 195 and the line frequency ripple to provide a substantially DC output voltage. Capacitor 125 is appropriately sized to prevent the high frequency switching of chip 195 from affecting the input current 197.
The values of capacitor 180, resistors 160, 165, and 170 determine the dominant pole of the power factor correction stage 120. The dominant pole is generally set at approximately 10 Hz. The frequency of the dominant pole is used to maintain the feedback current 192 at a constant level within an AC Mains half cycle of 100 Hz. Maintaining current 200 at a constant level would create a duty cycle which is constant in each AC Mains half cycle which in turn would generate an average MOSFET current 198 flowing through chip 195 that would itself be sinusoidal. As such, the input current 197 would not be sinusoidal since it would be the sum of a sinusoidal average MOSFET current 198 and a non-sinusoidal average diode current 199. To compensate for the fact that a constant feedback current 192 would create an average MOSFET current 198 that is sinusoidal, a feed forward scheme is employed using a feed forward current 191 through resistor 190 that varies the duty cycle linearly with the instantaneous rectified AC line voltage magnitude as can be seen in FIG. 5. This way the average MOSFET current 198 is distorted and the resulting input current 197 is maintained as sinusoidal.
The flyback converter stage 20 is operated like a conventional flyback converter stage with a second device using optocoupler feedback. The flyback converter can also be replaced with a forward converter.
Power Factor Correction stages of the type described with respect to FIG. 3 have been able to achieve power factors in excess of 0.95 and Total Harmonic Distortions less than 9%. However, the addition of a power factor correction stage is costly in terms of components which results in a far larger bill of materials and power supply size, as can be seen by comparing the number of components in FIG. 1 to the number of components in FIG. 3. Further, the additional components decrease the reliability of the power supply.
Another problem associated with conventional power supplies is that the power supplies operate and draw power even when the device that they are supplying is in a "sleep mode". In a computer, a conventional power supply will operate drawing large amounts power even if the computer is utilizing only minimum power in "standby" or "sleep" mode. By drawing a large amount of power when the system is not operating, power costs to the end user increases without deriving any appreciable benefits. When in sleep mode different components of a computer utilize a different amount of power. For instance the monitor and keyboard need very little power to continue operation, while the random access memory may need more power than the keyboard or monitor.
It is therefore desired to create a power supply that can supply power to a computer that can supply power for the differing needs of the different systems of the computer.
It is additionally desired to create a power supply that has standby functionality to decrease power consumption and associated costs for end users.